Formation of nanowhiskers on a substrate of dissimilar material

ABSTRACT

A method for forming a nanowhisker of, e.g., a III-V semiconductor material on a silicon substrate, comprises: preparing a surface of the silicon substrate with measures including passivating the substrate surface by HF etching, so that the substrate surface is essentially atomically flat. Catalytic particles on the substrate surface are deposited from an aerosol; the substrate is annealed; and gases for a MOVPE process are introduced into the atmosphere surrounding the substrate, so that nanowhiskers are grown by the VLS mechanism. In the grown nanowhisker, the crystal directions of the substrate are transferred to the epitaxial crystal planes at the base of the nanowhisker and adjacent the substrate surface. A segment of an optically active material may be formed within the nanowhisker and bounded by heterojunctions so as to create a quantum well wherein the height of the quantum well is much greater than the thermal energy at room temperature, whereby the luminescence properties of the segment remain constant without quenching from cryogenic temperatures up to room temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.11/165,126, filed Jun. 24, 2005, which issued on May 5, 2009 as U.S.Pat. No. 7,528,002 which claims the benefit of the priority of U.S.Provisional Patent Application No. 60/582,513, filed Jun. 25, 2004, theentirety of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to structures produced by techniques ofnanotechnology, and methods of producing such structures.

More specifically, the invention relates to such structures and devicesincorporating at least one element, essentially in one-dimensional form,which is of nanometer dimensions in its width or diameter, which isproduced with the aid of a catalytic particle, and which is commonlytermed a “nanowhisker.”

The invention relates also to a method of forming a nanowhisker of acertain material on a substrate of a dissimilar material.

2. Brief Description of the Prior Art

Nanotechnology covers various fields, including that of nanoengineering,which may be regarded as the practice of engineering on the nanoscale.This may result in structures ranging in size from small devices ofatomic dimensions, to much larger scale structures—for example, on themicroscopic scale. Typically, nanostructures are devices having at leasttwo dimensions less than about 1 μm (i.e., nanometer dimensions).Ordinarily, layered structures or stock materials having one or morelayers with a thickness less than 1 μm are not considered to benanostructures. Thus, the term nanostructures includes free-standing orisolated structures that have two dimensions less than about 1 μm, thathave functions and utilities different from those of larger structures,and that are typically manufactured by methods different fromconventional procedures for preparing somewhat larger, i.e., microscale,structures. Although the exact boundaries of the class of nanostructuresare not defined by a particular numerical size limit, the term has cometo signify such a class that is readily recognized by those skilled inthe art. In many cases, an upper limit of the size of the at least twodimensions that characterize nanostructures is about 500 nm. In sometechnical contexts, the term “nanostructure” is construed to coverstructures having at least two dimensions of about 100 nm or less. In agiven context, the skilled practitioner will recognize the range ofsizes intended. In this application, the term “nanostructure” is broadlyintended to refer to an elongated structure having at least twotransverse dimensions less than about 1 μm, as indicated above. In morepreferred applications, such dimensions will be less than about 100 nm,more preferably less than about 50 nm, and even more preferably lessthan about 20 nm.

Nanostructures include one-dimensional nanoelements, essentially inone-dimensional form, that are of nanometer dimensions in their width ordiameter, and that are commonly known as nanowhiskers, nanorods,nanowires, nanotubes, etc.

The basic process of whisker formation on substrates by the so-calledVLS (vapor-liquid-solid) mechanism is well known. A particle of acatalytic material, usually gold, is placed on a substrate and heated inthe presence of certain gases to form a melt. A pillar forms under themelt, and the melt rises up on top of the pillar. The result is awhisker of a desired material with the solidified particle meltpositioned on top. See Wagner, Whisker Technology, Wiley, New York,1970, and E. I Givargizov, Current Topics in Materials Science, Vol. 1,pages 79-145, North Holland Publishing Company, 1978. In earlyapplications of this technique, the dimensions of such whiskers were inthe micrometer range, but the technique has since also been applied forthe formation of nanowhiskers. For example, International PatentApplication Publication No. WO 01/84238 (the entirety of which isincorporated herein by reference) discloses in FIGS. 15 and 16 a methodof forming nanowhiskers, wherein nanometer sized particles from anaerosol are deposited on a substrate and these particles are used asseeds to create filaments or nanowhiskers.

Although the growth of nanowhiskers catalyzed by the presence of acatalytic particle at the tip of the growing whisker has conventionallybeen referred to as the VLS (Vapor-Liquid-Solid process), it has come tobe recognized that the catalytic particle may not have to be in theliquid state to function as an effective catalyst for whisker growth. Atleast some evidence suggests that material for forming the whisker canreach the particle-whisker interface and contribute to the growingwhisker even if the catalytic particle is at a temperature below itsmelting point and presumably in the solid state. Under such conditions,the growth material, e.g., atoms that are added to the tip of thewhisker as it grows, may be able to diffuse through a the body of asolid catalytic particle or may even diffuse along the surface of thesolid catalytic particle to the growing tip of the whisker at thegrowing temperature. Persson et al., “Solid-phase diffusion mechanismfor GaAs nanowires growth,” Nature Materials, Vol. 3, October 2004,pages 687-681, shows that, for semiconductor compound nanowhiskers theremay occur solid-phase diffusion mechanism of a single component (Ga) ofa compound (GaAs) through a catalytic particle. Evidently, the overalleffect is the same, i.e., elongation of the whisker catalyzed by thecatalytic particle, whatever the exact mechanism may be under particularcircumstances of temperature, catalytic particle composition, intendedcomposition of the whisker, or other conditions relevant to whiskergrowth. For purposes of this application, the term “VLS process,” or“VLS mechanism,” or equivalent terminology, is intended to include allsuch catalyzed procedures wherein nanowhisker growth is catalyzed by aparticle, liquid or solid, in contact with the growing tip of thenanowhisker.

For the purposes of this specification the term “nanowhisker” isintended to mean a one-dimensional nanoelement with a width or diameter(or, generally, a cross-dimension) of nanometer size, the elementpreferably having been formed by the so-called VLS mechanism, as definedabove. Nanowhiskers are also referred to in the art as “nanowires” or,in context, simply as “whiskers” or “wires,”

Several experimental studies on the growth of nanowhiskers have beenmade, the most important reported by Hiruma et al. They grew III-Vnanowhiskers on III-V substrates in a metal organic chemical vapordeposition (MOCVD) growth system. See Hiruma et al., J. Appl. Phys. 74,page 3162 (1993); Hiruma et al., J. Appl. Phys 77, page 447 (1995);Hiruma et al., IEICE Trans. Electron., E77C, page 1420 (1994); Hiruma etal., J. Crystal Growth, 163, pages 226-231 (1996).

More recently, growth of Si nanowires on Si substrates has beendemonstrated. See, e.g., Westwater et al., J. Vac. Sci. Technol., B1997, 15, page 554. Very recently, growth of Ge nanowires on Sisubstrates was also demonstrated. See Kamins et al., Nano Lett. 2004, 4,pages 503-506, Web published Jan. 23, 2004.

In the prior art in general, many different approaches have been triedin order to realize perfect epitaxial growth of III-V materials onsilicon substrates. The primary motivation for these strong efforts isthat, if such a technology could be developed, a very wide spectrum ofso called III-V heterostructure devices may be incorporated withmain-stream silicon technology, thus opening the way to highly advancedhigh-speed and opto-electronic devices incorporated with silicon.

Besides the efforts toward integrating III-V materials on Si, otherapproaches toward the specific goal of efficient light-emission using Sihave been proposed—for example, the formation of porous Si viaelectrochemical etching (Canham, L. T., Appl. Phys. Lett., 1990, 57,page 1046) and the incorporation of luminescent defects, such asrare-earth impurities (Michel et al, Semiconduct. Semimet., 1998, 49,page 111.

Epitaxial growth of III-V semiconductors on Si presents a number ofdifficulties, such as lattice mismatch, differences in crystal structure(III-V's have a polar zinc blende or wurtzite structure whereas Si has acovalent diamond structure), and a large difference in thermal expansioncoefficient. Much work has been done on planar growth of layers of III-Vmaterials on Si substrates using different approaches such as bufferlayers, growth on patterned Si surfaces, and selected area growth fromsmall openings. See, for example, Kawanami, H., Sol. Energy Mater. Sol.Cells, 2001, 66, page 479.

A major challenge has been to avoid the formation of antiphase domainsrelated to the initiation of III-V growth on two atomic planes ofsilicon differing by one atomic layer, which leads to the formation ofanti-phase domain walls and defective material. In Ohlsson et al.,“Anti-domain-free GaP, grown in atomically flat (001) Si sub-μm-sizedopenings”, Appl. Phys. Lett., Vol 80, No. 24, 17 Jun. 2002, pages4546-4548, to address the problem of antiphase domains, GaP nanocrystalswere grown on a Si(001) substrate surface through openings in a mask ofSiO₂. The mask openings were defined by e-beam lithography. Etching andchemical stripping followed, and organic residues were removed by oxygenplasma. A high annealing temperature of 1000° C. was used to remove Sioxide and to provide atomic flatness on the silicon surface on which GaPis nucleated. An atomically flat surface is a surface that presents asingle crystal facet and does not exhibit atomic steps.

In prior U.S. patent application Ser. No. 10/613,071, published as No.2004-0075464, to Samuelson et al., and International Patent ApplicationPublication No. WO-A-04/004927 (both of which publications areincorporated herein by reference), there are disclosed methods offorming nanowhiskers by a chemical beam epitaxy method. Nanowhiskers aredisclosed having segments of different materials, with abrupt or sharpheterojunctions therebetween. Structures are disclosed comprisingnanowhiskers of, for example, gallium arsenide extending from a siliconsubstrate. Processes are disclosed for forming epitaxial layers of III-Vmaterials on a silicon substrate, involving initial formation ofnanowhiskers on the substrate and using the nanowhiskers as nucleationcenters for an epitaxial layer.

Improvements are desirable in the formation of nanowhiskers of III-Vmaterials (or having at least a base portion of III-V material) on asubstrate of Group IV material to ensure that the nanowhiskers are grownin a highly reliable way with accurate predetermined dimensions andstructure, and with accurately predetermined physical characteristicsfor implementing the above structures and processes. More generally,improvements are desirable in the formation of nanowhiskers having atleast a base portion of a predetermined material on a substrate of adissimilar material.

SUMMARY OF THE INVENTION

One object of the invention is to provide a method for forming, on asubstrate of a Group IV material, a nanowhisker having at least a baseportion of a III-V semiconductor material.

A more general object of the invention is to provide a method forforming a nanowhisker, having at least a base portion of a predeterminedmaterial, on a substrate of a dissimilar material.

A further object of the invention is to provide a nanostructurecomprising a nanowhisker upstanding form a substrate of a Group IVmaterial and having at least a base portion of a III-V semiconductormaterial.

A further object of the invention is to provide a nanostructureincluding a nanowhisker formed on a substrate of a Group IV material,the nanowhisker having at least a base portion of a III-V semiconductormaterial and one or more segments of a further material bounded withabrupt or sharp heterojunctions.

In accordance with a first aspect, the invention provides a method forforming a nanowhisker, having at least a base portion of a firstmaterial, on a substrate of a second material, different from said firstmaterial, comprising:

providing a substrate of said second material having a surface that isprepared to remove impurities and to provide at least one atomicallyflat growth region;

providing on said growth region at least one catalytic particle;

introducing into the atmosphere surrounding the substrate, gases forforming the nanowhisker, and heating the substrate to a predeterminedgrowth temperature at which the nanowhisker is grown on said growthregion via a said catalytic particle.

In accordance with a more specific aspect, the invention provides amethod for forming a nanowhisker, having at least a base portion of aIII-V first semiconductor material on a substrate of a Group IV secondmaterial, comprising:

providing a substrate of said second material having a surface that isprepared to remove impurities and to provide at least one atomicallyflat growth region;

providing on said growth region at least one catalytic particle; and

introducing into the atmosphere surrounding the substrate, gases forforming the nanowhisker, and heating the substrate to a predeterminedgrowth temperature at which the nanowhisker is grown via a saidcatalytic particle.

It has been found that an issue in achieving growth of III-Vnanowhiskers (i.e., nanowhiskers of which at least the initial growth orbase portion is of a III-V material) on a silicon substrate, or otherGroup IV material, is to provide essentially a perfect surface fromwhich catalytic growth is initiated. In order to provide for theformation of an ideal group III-V (or Group II-VI) semiconductoron thesubstrate, the interface between the metallic nanoparticle and thesubstrate has to be of a character that only a single-domain of thecrystalline nanowhisker material is formed. This may ideally be obtainedby keeping the interface atomically flat over the diameter of thenanowire, or more generally by nucleation conditions that lead to asingle nucleus from which the nanowhisker nucleates and grows. Thus, itis desirable to have a totally clean surface that is free from impurityand oxide and that is preferably also atomically flat at the whiskergrowth site (or at least sufficient that the nanowhisker nucleates andgrows from a single nucleus). In this way, growth of the nanowhiskermaterial in bulk form that might occur at imperfections on the substratesurface is inhibited, and factors that promote nanowhisker growth areencouraged, such as migration of nanowhisker material along the surfaceof the substrate to the catalytic growth particle.

In practice of the invention, it is desirable to employ one or morecleaning operations to remove organic residue that is commonly found ona substrate surface. It is also desirable to employ steps such asetching to remove existing oxide formations on the substrate surface. Inorder to prevent further oxide growth on the substrate surface,subsequent to these removal steps, it is desirable to passivate thesurface, at least temporarily, such as by HF etching.

It would in principle be possible to provide a surface with a layer ofpassivating material such as silicon nitride or silicon dioxide, and toform apertures in the passivating layer (for example, by etching) and totreat the exposed substrate surface in the apertures to achieve theideal conditions as described above. Catalytic particles would then beformed within respective apertures by a suitable deposition process.

In a currently preferred embodiment, however, the entire substratesurface is processed towards an ideal condition, followed by depositionon the surface of catalytic particles by an aerosol process. To thisend, it is preferred to passivate the surface by means of etching withan acid such as HF. This has the effect that free or dangling bonds onthe substrate surface are terminated with hydrogen ions. This preventsfurther oxide growth on the substrate surface, and maintains idealsurface conditions while aerosol deposition takes place. Aerosoldeposition, and any further processing on the substrate surface takesplace in accordance with the invention before any significantdegradation of the passivation properties of the hydrogen termination.In practice, a time period of about 2 hours may be permissible.

As regards the catalytic particles, any metal that is commonly used fornanowhisker growth, such as Au, may be used with this invention. Inparticular applications where it is not desirable to use Au on siliconsubstrates, because of the tendency for Au to diffuse into the siliconand create deep level defects, other materials such as In or Ga may beused, where the nanowhisker to be formed contains such materials.

The catalytic particles are preferably provided on the substrate surfacein the form of an aerosol deposition, as noted above. This has anadvantage that very accurate control may be exerted over the size of theparticles (see International Patent Application Publication No. WO01/84238, the entirety of which is incorporated herein by reference).Alternatively, the catalytic particles may be formed by deposition froma liquid suspension (colloids from a solution), or may be defined by aNIL (nano imprint lithography) process. In yet another alternativemethod of forming catalytic particles, a thin film of catalytic materialmay be formed over substrate surface in a manner similar to thatdisclosed in the above-referenced publications to Hiruma et al. When thesubstrate is heated in an initial annealing step, the film liquifies andbreaks up into catalytic particles, from which nanowhiskers may begrown.

It has been found that in order to achieve epitaxial nanowhisker growth,it is desirable to anneal the substrate surface prior to nanowhiskergrowth. Such annealing preferably occurs at a temperature between 600°C. and 650° C. for a silicon surface. Further, it has been found thatgases containing elements for nanowhisker growth should not be presentin the atmosphere surrounding the substrate during this annealingprocess. This is in contrast to prior procedures where it is common toexpose the substrate during the annealing process to a gas containingGroup V elements, such as arsenic or phosphorous.

VLS growth may occur by the chemical beam epitaxy method or MOCVD(MOVPE) method. In these methods, it is common to employ two sources ofgas, one being a organometallic compound containing a metal, such asgallium or indium, that is required to form part of the nanowhiskermaterial, and another gas containing a Group V or VI element, such asphosphine or arsine, that is desired to react with said metal to producethe compound of the nanowhisker.

In MOCVD techniques, it is common to introduce the gases together forgrowth. In addition, it is common to have the phosphine, arsine orsimilar hydride gas, introduced during the annealing step. However, incontrast to this, it has been found in accordance with the presentinvention that such gas should not be introduced during the annealingstep.

With the above measures, it has been found that very accuratelydetermined nanowhiskers having precise dimensions and physicalcharacteristics can be formed on substrates of dissimilar material. Inparticular, materials of a III-V semiconductor material, such as indiumphosphide, gallium arsenide, gallium phosphide etc., may be formed onGroup IV material substrates such as silicon. Silicon substrates areespecially preferred in practice of the invention, in that suchsubstrates are inexpensive and commonly used in industry, as opposed tosubstrates of III-V materials that are much more expensive. Any surfaceof the silicon or other substrate material may, in principle, be usedfor whisker growth in practice of the invention, e.g. (001), (111).

It has been found that the crystalline perfection of nanowhiskers grownin accordance with the invention is such that, at the base of thenanowhisker at its junction with the substrate, crystal planes areepitaxially formed within the nanowhisker one upon the other, to form atransition of crystal material where the crystalline orientation of thesubstrate surface is preserved in the epitaxially grown layers of thewhisker. That is to say, there is not an indistinct region of amorphousmaterial and crystal segments at the base, between the substrate and thealigned crystal planes of the nanowhisker. Defects or dislocations mayexist at the base, e.g. stacking faults, but these are not such as todisturb the essential crystallinity and the crystal directions of thesubstrate surface that are transferred to the epitaxial growth of thewhisker. The problems of lattice mismatch are largely accommodated byradial expansion of the diameter of the base of the nanowhisker.

Accordingly, in yet another aspect, the invention provides ananostructure comprising a nanowhisker upstanding from a substrate ofsilicon and having at least a base portion formed of a III-V material,wherein at a junction of the base portion with the substrate, crystalplanes are epitaxially formed within the nanowhisker one upon the other,and crystallographic directions of the substrate surface are transferredto the epitaxially formed crystal planes of the nanowhisker.

It will be appreciated that with the present invention, structures andprocesses that have been previously proposed and demonstrated (forexample, in the aforementioned U.S. patent application Ser. No.10/613,071, published as No. 2004-0075464, and International PatentApplication Publication No. WO-A-04/004927) may be more accurately andreliably implemented. In particular, the processes disclosed for formingepitaxial layers on silicon substrates may be implemented with a reducedrisk of non-epitaxial growth.

The present invention is in principle applicable to any of the materialsthat may be used in the manufacture of nanowhiskers and substratestherefor. Such materials are commonly semiconductors formed of Group IIthrough Group VI elements. Such elements include, without limitation,the following:

Group II: Be, Mg, Ca; Zn, Cd, Hg;

Group III: B, Al, Ga, In, Tl;

Group IV: C, Si, Ge, Sn, Pb;

Group V: N, P, As, Sb;

Group VI; O, S, Se, Te.

Semiconductor compounds are commonly formed of two elements to makeIII-V compounds or II-VI compounds. However, ternary or quaternarycompounds are also commonly employed involving, e.g., two elements fromGroup II or from Group III. Stoichiometric and non-stoichiometricmixtures of elements are commonly employed.

III-V materials and II-VI materials include, without limitation, thefollowing:

AlN, GaN, SiC, BP, InN, GaP, AlP, AlAs, GaAs, InP, PbS, PbSe, InAs,ZnSe, ZnTe, CdS, CdSe, AlSb, GaSb, SnTe, InSb, HgTe, CdTe, ZnTe, ZnO.

In accordance with the invention, a substrate may be selected from oneof the above Group IV, III-V or II-VI materials, and the nanowhiskers(or at least the base portions thereof) may be selected from another ofthe Group IV, III-V or II-VI materials. Thus, the substrate may inprinciple be of Group IV, Group III-V or Group II-VI material, and thenanowhiskers may similarly be materials within these groups. Othersubstrates that are commonly used, such as aluminium oxide (sapphire) orsilicon carbide, may also be employed in principle. In its mostpreferred practice, however, the invention is specifically concernedwith substrates of silicon or other Group IV materials that are commonlyavailable in the industry, and nanowhisker compounds of III-V material.

It will be understood that there may have been proposed and demonstratedother methods for producing nanowhiskers of III-V material on Group IVmaterial, particularly those Group IV materials that are easier to workwith than silicon. However, it is generally recognized that silicon isthe most difficult of the Group IV materials to work with. The presentinvention achieves successful growth of III-V nanowhiskers on silicon,and may readily be applied to Group IV and other substrate materialsgenerally, without the exercise of inventive ingenuity.

As one of its principal advantages, the invention demonstrates epitaxialnucleation and growth of III-V semiconductor nanowhiskers on siliconsubstrates. This addresses the long-time challenge of integrating highperformance III-V semiconductors with mainstream silicon technology.

The present invention additionally permits the formation of lightemitting or light detecting devices within nanowhiskers grown inaccordance with the principles herein described. In prior U.S. patentapplication Ser. No. 10/613,071, published as No. 2004-0075464, toSamuelson et al., and International Patent Application Publication No.WO-A-04/004927, there are disclosed various light emitting and lightdetecting devices incorporated within a nanowhisker. For example, a p-njunction may be formed by doping two adjacent segments of thenanowhisker with oppositely charged dopant ions. This may be used as alight emitting diode, or as a photodetector. A heterojunction betweentwo segments of different material within a nanowhisker may providesimilar functions to a p-n junction. A segment of an optically activematerial forming heterojunctions with adjacent portions of thenanowhisker may form laser devices, resonant tunneling diodes,heterobipolar transistors, and other electronic and photonic devices.Further there is disclosed a method of forming heterojunctions within ananowhisker, which are accurately formed and may extend over a width ofbetween one and eight atomic layers of the nanowhisker crystal. Suchheterojunctions may be atomically abrupt, extending over as few as oneor two atomic layers. As stated in the above referenced application U.S.patent application Ser. No. 10/613,071, “sharp heterojunction” means atransition from one material to another material over five or lessatomic monolayers. However, for the purpose of the presentspecification, heterojunctions may extend over five or more than fiveatomic monolayers, but yet still provide a desired function of quantumconfinement and defining a quantum well. Heterojunctions that define aquantum well may, in this specification, be referred to as “sharp.”

In a further aspect, the invention permits the incorporation of doubleheterostructure segments in such nanowhiskers, allowing efficientroom-temperature generation of light from, e.g., III-V nanowires grownon Si substrates. Advanced heterostructure devices presently realized onvery expensive, silicon-incompatible III-V substrates, such as resonanttunneling diodes, superlattice device structures and heterostructurephotonic devices for on-chip communication, are, in accordance with theinvention, available as complementary device technologies forintegration with silicon.

Thus, in another aspect, the invention is concerned with a nanowhiskerhaving at least a base portion of a III-V semiconductor material, andthe nanowhisker being formed on a substrate of a Group IV material andincluding a segment of a further material disposed along the length ofthe nanowhisker, wherein accurately formed heterojunctions are providedat the boundaries of the segment with adjacent portions of thenanowhisker.

It has been found that such heterojunctions may be very precisely andaccurately formed with sharp or abrupt junctions extending over only afew atomic lattice planes. The material of the nanowhisker in general issingle crystal, single domain, pure epitaxial growth without defect ordislocation. It has been found that the optical characteristics of suchnanowhiskers are very high quality, with the luminescence propertiesremaining constant from very low cryogenic temperatures up to roomtemperatures, without quenching of the luminescence. This arises, insignificant part, because the confining potential of the quantum wellformed by the heterojunctions bounding the segment is well defined (atleast 200 meV up to 500 meV) and much greater than the thermal energy atroom temperature, kT (˜25 meV), so that the thermal movement of freecharge carriers does not disturb the energy distributions of thecarriers within the quantum well.

Thus, in a further aspect, the present invention provides a structure,including a nanowhisker formed on a substrate of a Group IV material,the nanowhisker having at least a base portion of a III-V semiconductormaterial and a segment of a further material disposed along the lengthof the nanowhisker, wherein accurately formed heterojunctions areprovided at the boundaries of the segment with adjacent portions of thenanowhisker such as to create a quantum well bounding the segment,wherein the height of the quantum well is much greater than the thermalenergy at room temperature so as to provide a device that is one of aphotonics device and an electronics device.

Further, the luminescence properties of the segment remain essentiallyconstant, with substantially no quenching, from cryogenic temperaturesup to room temperature, thereby providing for highly reliable photonicsdevices. In accordance with the invention, the provision of nanowhiskerscontaining optically active material on a silicon substrate permits avery effective implementation of optical interconnects on a siliconchip. As processor speeds increase, a limiting factor in silicon chipsis speed of transmission of optical pulses along buses formed asconductive lines on the substrate. A means of avoiding this limitationis to provide a data bus in the form of an optical interconnectcomprising an optical path including a light emitter and a lightreceiver positioned on the substrate surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the accompanying drawings.

FIGS. 1A to 1C show a first embodiment of the invention, comprisinggrowth of GaP nanowhiskers on Si (111). FIG. 1A is a 45° tilt SEM imageof GaP nanowires growing vertically from the Si (111) surface in the[111] direction. FIG. 1B shows a top view from the same sample, scalebar 1 μm. FIG. 1C is an HRTEM image of the Si substrate-nanowhiskerinterface of a nanowhisker of FIG. 1A, scale bar 10 nm. The crystaldirections from the Si substrate (lower) are transferred to thenanowhisker (upper).

FIGS. 2A and 2B show modifications of the first embodiment, specificallySEM images of vertical GaAs (FIG. 2A) nanowhiskers and InP (FIG. 2B)nanowhiskers grown from and Si (111) substrate, tilt 45°, scale bars 1μm.

FIGS. 3A and 3B show a second embodiment of the invention, HAADF STEMimages of a light-emitting GaAsP segment incorporated in a GaP nanowireduring growth. The segment is approximately 500 nm long corresponding toa growth time of 1 min of a total of 5 min. In FIG. 3A, the location ofthe segment in the nanowire is seen as a brighter region in the midsection, scale bar 500 nm. FIG. 3B is an XEDS line scan of the GaPnanowire with GaAsP segment showing the sharp nature of the interface,scale bar 200 nm.

FIGS. 4A to 4C show photoluminescence from the nanowhiskers of FIG. 3.FIG. 4A shows room-temperature PL from standing wires as-grown on aSi(001) surface as seen from above, scale bar 5 μm. When excited with a458 nm laser source the wires emit at 725 nm. The luminescence wasvisible to the naked eye and the image was recorded using a standarddigital camera with 15 s integration time. FIG. 4B shows a top view SEMimage of the same sample, scale bar 1 μm. Four nanowhiskers grow in thefour different <111> directions. The nanowhiskers form an angle of 35.3°with the Si(001) surface as illustrated in the inset. FIG. 4C shows 10 Kand room-temperature PL spectra from individual wires scraped off andresting on a SiO₂ surface. The luminescence from the wires remainedbright at room temperature, with negligible quenching observed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The integration of III-V compound semiconductors, which are dominant inapplications such as light-emitting diodes and optoelectronics, withmainstream Si technology is a long sought-after goal for thesemiconductor industry. If mastered, significant limitations of theotherwise ideal Si material could be compensated: first, the lowefficiency in light generation in Si and, second, the lack of aversatile heterostructure technology required for many high-speedelectronic and photonic devices.

The present invention, in an especially preferred mode, provides III-Vnanowhiskers (i.e., nanowhiskers of which at least the initial growth orbase portion is of a III-V material) grown epitaxially on Si substrates.By the term “epitaxially,” it is meant that the crystallographicdirections are transferred from the substrate to the nanowhiskers. GaPhas a lattice mismatch of less than 0.4% relative to Si and is thereforea preferred candidate for epitaxial growth on Si among the III-Vcompounds. The GaP—Si junction has applications in heterojunctionbipolar transistors with GaP as large band gap emitter with sharp andideal interfaces to Si. Successful synthesis is demonstrated ofepitaxially oriented GaP nanowhiskers on Si(111) and Si(001) substrates.To demonstrate room temperature light generation on silicon, lightemitting GaAsP heterostructure segments were inserted. The presentinvention provides epitaxial growth of nanowhiskers on Si for moreheavily lattice-mismatched compounds such as InP (4.1%) and GaAs (8.1%).

In a first embodiment, size-selected gold aerosol nanoparticles wereused as seeding particles for nanowire growth. Prior to aerosoldeposition, the Si substrates were cleaned and organic residues removed.As a final step before deposition, the samples were treated withhydrofluoric acid to create a hydrogen-terminated surface. The sampleswere then immediately transferred to a controlled nitrogen atmospherewhere the aerosol deposition took place. Typically, 40 nm diameter Auaerosol particles at a density of 2 μm⁻² were used. After aerosoldeposition, the sample was exposed as little as possible to open airsince the hydrogen-terminated surface is known to deteriorate with time.The nanowire growth was performed in a low-pressure, 10 kPa, MOVPEsystem. Samples were annealed at 625° C. in a hydrogen atmosphere for 10min before growth. The temperature was then ramped down to the growthtemperature of typically 475° C. Growth of GaP nanowhiskers wasinitiated when the precursors, trimethyl gallium and phosphine, wereintroduced simultaneously into the growth cell. A typical growth timewas 4 min. For incorporation, in a second embodiment of the invention,of an optically active GaAsP heterosegment, arsine was switched on at acertain time during growth. The GaAs_(x)P_(1-x) composition was thencontrolled by adjusting the arsine-to-phosphine ratio. For growth of InPand GaAs on Si, the procedure was very similar but with differenttemperatures and precursors as appropriate to those materials.

Samples were then characterized using scanning electron microscopy(SEM), transmission electron microscopy (TEM), and photoluminescence(PL) spectroscopy. Specifically, FIG. 1A shows a 45° tilt SEM micrographof GaP nanowhiskers growing vertically from the Si(111) surface in the[111] direction. A thin planar film of GaP on the Si substrate can beseen as a corrugation of the surface between the wires. TEMinvestigations estimate the film thickness to be about 20 nm, i.e., theuncatalyzed planar growth rate is approximately 10-2 of the nanowiregrowth rate. The wires were grown using 40 nm seed Au nanoparticles. Topwire diameter is close to 40 nm. FIG. 1B is a top view of the samesample showing the perfection in the vertical alignment. FIG. 1C is aHRTEM image of the Si substrate-GaP nanowhisker interface. The crystaldirections from the Si substrate are transferred to the nanowhisker.

The preferred III-V nanowhisker growth direction in most reported casesin the literature is the [111]B direction, i.e., corresponding tovertical growth from a (111) oriented surface. The Si(111) surfaceactually has four possible <111> growth directions, one vertical andthree forming an angle of 19.5° with the substrate surface, distributed120° apart azimuthally. Only the vertical [111] direction is observed,which is expected if the gold-silicon interface is flat, as the onlyfacet available for nucleation is then the (111) facet, the other facetssimply not being present during nucleation. This is indisputably thecase when looking at FIGS. 1A and B and clearly demonstrates the perfectepitaxial nature of the growth. Well-aligned vertically orientednanowhiskers were reproducibly obtained in a large number (20+) ofgrowth runs.

To investigate the interface between the Si substrate and the GaPnanowire, samples were prepared for high-resolution transmissionelectron microscopy (HRTEM) by cleaving polishing, and ion milling thesilicon substrate after wire growth (FIG. 1C). The transfer ofcrystallographic information from the Si substrate to the GaPnanowhiskers can clearly be seen, in that the crystal directions of theSi substrate are transferred to the epitaxial layers at the base of thenanowhisker.

In modifications of the first embodiment, III-V compounds with a largelattice mismatch such as GaAs (FIG. 2A) and InP (FIG. 2B), with latticemismatch of 4.1% and 8.1% respectively, are also be grown epitaxially onSi. Specifically, FIGS. 2A and 2B show SEM images of vertical (A) GaAsnanowhiskers and (B) InP nanowhiskers grown on Si(111) substrates. Thesmall wire cross-section enables the wires to accommodate and relaxstrain from the large lattice misfits of otherwise incompatiblematerials.

It was found that passivation (e.g., hydrogen passivation) of the Sisurface is particularly advantageous. On samples where the native oxidewas not removed prior to aerosol deposition, no epitaxial orientationwas observed. It was also noticed that for samples that were kept in aglovebox atmosphere for a longer time (˜3 months), the yield of straightepitaxial wires was lower than from freshly prepared samples. As thereoxidation of the HF-etched surface is moderately slow, this suggeststhat even a very thin layer of native oxide is detrimental to epitaxialwire growth.

Examples

Starting material was “Toyo” Epitaxial Silicon Wafers, P type substratewith a P type epilayer orientation (111).

Dopant Resistivity Thickness Substrate Boron <0.015 Ω cm 245 μm LayerBoron    12 Ω cm  27 μm

Preparation Prior to Growth

1) Clean with ultrasonic Tri-clean to remove organic residues andparticles.

The wafer was placed in a test tube and solution a) below was added. Thetest tube was put in ultrasonic bath at 35 KHz for 2-3 minutes. After,the bath the solution was decanted and the next solution was added. Theprocess repeated for solutions b)-d) in that order.

a) Trichloroethylene (proanalysi)

b) Acetone (proanalysi)

c) Ethanol (95%)

d) Milli-Q-H₂O

The water was decanted and refilled 2-3 times in Milli-Q-H₂O (18.2 W cm25° C.). Rinse the wafer.

2) The wafer was removed from the rinse water and immediately cleanedwith Piranha Etch to remove any remaining organic residues.

The following were measured and mixed in a separate container:

-   -   7 parts sulphuric acid (95-97% proanalysi)    -   3 parts hydrogen peroxide (30% proanalysi)

When mixed, an exothermic reaction causes the solution to heat to over70° C. This mixture was poured over the samples and was stirredoccasionally for 6 min.

The Piranha Etch was decanted and the wafer was rinsed as before 3-4times.

3) The wafer was taken directly from the rinsing water and put in aHydrofluoric acid (HF) dip to remove silicon dioxide on the surface.

A 5% HF solution was prepared by measuring and mixed in a separatecontainer:

-   -   1 part HF (40% proanalysi)    -   7 parts in Milli-Q-H2O (18.2 M W cm 25° C.)

The solution was stirred occasionally for 2 min.

The wafer was removed from the HF solution and care was taken that therewere no visible droplets remaining on the polished side of the wafer.The backside of the wafer was blotted on a filter paper to remove anyliquid on the backside of the wafer. It was then transferred directlyinto an atmospherically controlled glove box (H₂O and O₂ levels <1 ppm)via a load lock for the aerosol deposition.

A standard aerosol particle diameter of 40 nm was used with particlesurface densities ranging from ˜0.05 to 40 μm⁻².

After aerosol deposition, the samples were stored up to 2 weeks in anatmospherically controlled glove box until they were transferred in airto the MOVPE glove box chamber for loading and growth.

The samples were then mounted in the growth chamber of the MOVPE system(low pressure 100 mbar).

A typical growth run, GaP nanowhiskers:

1. Temperature was raised to an annealing temperature of 625° C. andannealed for 10 min under hydrogen atmosphere. Temperature was rampeddown linearly during 5 min to growth temperature, 475° C.

2. Growth started when the two sources, TMG and phosphine weresimultaneously introduced in the growth chamber. The molar fractionsource flows were 1.5×10⁻² for phosphine and 1.25×10⁻⁵ for TMG in 6l/min hydrogen. A typical growth time was 4 minutes.

3. Growth stops when the TMG is switched off. The temperature is thenlowered and the phosphine is switched off as temperature drops below300° C.

Comments to the procedure described above:

-   -   The HF-etch creates a hydrogen-terminated surface, i.e., a        hydrogen atom is attached to each dangling bond of the Si (111)        surface. Other surface preparations such as no cleaning at all,        organic clean but no oxide removal, did not produce good wire        growth. As a hydrogen terminated surface is oxidized over time,        it is preferable to use freshly prepared samples. Samples kept        in a glove box atmosphere for ˜3 months produced lower quality        wires than freshly prepared samples.    -   Annealing temperature was found to be an important parameter for        the wire quality and investigated temperatures in the range 550        to 700° C. A high annealing temperature (700° C.) gave wires        with a heavy base and irregular nucleation, resulting in many        small wires around the main stem as well as many wires creeping        along the surface with no orientation. A low annealing        temperature (550° C.), on the other hand, resulted in loss of        the epitaxial orientation from the substrate, i.e., the wires        were no longer vertically aligned but had a random orientation.        At this low temperature, many gold particles also did not        nucleate to form wires but remained as dead particles lying on        the surface. 625° C. was found to be a suitable compromise        between the two extremes above.    -   It was observed that if the phosphine was activated during the        annealing step, as is the conventional procedure when growing        GaP nanowhiskers on GaP substrates, there was no wire growth.

FIGS. 1A to 1C show GaP nanowhiskers grown on a silicon substrate. Theformation of the nanowhiskers is ideal, with the nanowhiskers exhibitingperfect regularity. In general, the achievement of ideal nanowhiskers isdue to the formation of perfect conditions for nanowire growth,including atomically flat surfaces with no impurity or oxide formationthat might give rise to bulk growth and that might inhibit factors thatpromote nanowhisker growth.

Referring to FIGS. 3A and 3B and 4A to 4C, in a second embodiment ofthis invention, light-emitting segments of GaAs_(x)P_(1-x) were insertedin GaP wires grown on Si. The composition can be tuned by controllingthe arsenic to phosphorus ratio during growth, and the length of thesegment is determined by the growth time.

The method of forming the nanowhiskers was essentially the same as thatin Example 1, but conditions are changed during growth to produce thegallium arsenide phosphide heterojunctions. The procedure for changingconditions is described in earlier mentioned U.S. patent applicationSer. No. 10/613,071, published as No. 2004-0075464, to Samuelson et al.

Using Si(001) substrates, the nanowhiskers grew in four different <111>directions (FIG. 4B). On the (001) surface orientation, four equivalent<111> directions make an angle of 35.3° with the substrate distributed90° apart azimuthally. For epitaxial growth, all four directions can beexpected since the <111> directions are equivalent.

FIG. 3A shows a high angle annular dark-field scanning transmissionelectron microscopy (HAADF-STEM) image of a GaP nanowire with a 500 nmlong segment of GaAs_(x)P_(1-x). An X-ray energy dispersive spectrometry(XEDS) composition line scan of the segment (FIG. 3B) shows that theinterfaces are very sharp. From XEDS composition analysis, a compositionof ˜30% P and ˜70% As in the segment can be inferred. The phosphorouscontent of the GaAsP segment measured with XEDS is probably somewhathigher than that of the actual segment core; after growth of thesegment, a thin shell of GaP is deposited over the GaAsP core due tolateral growth when the end part of the GaP nanowire is grown.

The optically active segments were characterized using PL spectroscopyand PL imaging. FIG. 4A shows room temperature luminescence imaging inthe deep red spectral region (725 nm) from standing wires, as grown onSi (001). The nanowhiskers were excited using an Ar+ laser, emitting at458 nm and with an intensity of approximately 3 kW/cm². A sample with alow wire density of ˜0.05 μm⁻² was used to make it possible to resolveindividual wires. The (001) substrate orientation was chosen to ease thecollection of the light since light is mainly emitted in lobes from thesegment and the light is collected from above. The elongations of thespots in two perpendicular directions correspond to the projection ofthe four different <111> directions (FIG. 4B). The fact thatluminescence of individual nanowhiskers can be imaged at roomtemperature suggests that the radiative recombination from GaP/GaAsP/GaPdouble heterostructure segments is not thermally quenched even at roomtemperature.

For a detailed PL-spectroscopy study, standing nanowhiskers were scrapedoff from a (111) substrate and transferred to a grid-patterned SiO₂surface. The advantage of placing the nanowhiskers on the grid structureis that, after PL spectroscopy, each wire can be located with SEM toconfirm that it is a single wire with a well-defined segment. PL spectrafrom separate nanowhiskers were recorded at 100K and room temperature,demonstrating high uniformity (FIG. 4C) in the luminescence from theindividual wires. The GaP/GaAsP/GaP nanowhiskers exhibit sharp peaks atabout 1.78 eV with a full width half-maximum (fwhm) of about 60 meV at10° K. The PL remains bright at room temperature with peaks shifted to1.71 eV and with an average fwhm of about 75 meV, with negligiblequenching of the emission. The spectral shift corresponds well with theband-gap shrinkage from 10° K to room temperature. Comparing the PLspectra with data in the literature for bulk GaAsP, a composition ofGaAs_(0.8)P_(0.2) can be inferred, in reasonable agreement with the XEDScomposition analysis. By changing the As_(x)P_(1-x) composition in thesegment, it is possible to continuously tune the emitting wavelengthfrom the band gap of GaP to the band gap of GaAs, representing awavelength span of 550-900 nm, corresponding to the spectral rangeachieved in GaAsP LED technology for growth on GaP.

Among its most important advantages, the present invention providesdevice-quality III-V semiconductor growth on silicon substrates withperfect epitaxial nucleation of oriented III-V nanowhiskers. The presentinvention additionally demonstrates visible room-temperatureluminescence of heterostructure III-V nanowhiskers formed on siliconsubstrates as bright as at cryogenic temperatures.

1. A method for forming a nanowhisker, having at least a base portion ofa first material, on a substrate of a second material different fromsaid first material, comprising: providing a substrate of said secondmaterial having a surface comprising at least one catalytic particle onan atomically flat growth region; and epitaxially growing thenanowhisker on said growth region via said catalytic particle, whereinsaid substrate comprises a Si(001) substrate, wherein said firstmaterial is selected from a group consisting of II-VI and III-Vsemiconductor compounds.
 2. A method according to claim 1, wherein saidfirst material is a III-V semiconductor compound.
 3. A method accordingto claim 1, wherein said at least one catalytic particle is depositedfrom an aerosol.
 4. The method of claim 1, wherein the nanowhiskercomprises a base adjacent the substrate and there is not a region ofamorphous material and crystal segments between the base and thesubstrate.
 5. The method of claim 4, wherein epitaxially growing thenanowhisker results in the catalytic particle being located at a growthtip of the nanowhisker, wherein the growth tip is at an opposite end ofthe nanowhisker from the base.
 6. A method for forming a nanowhisker,having at least a base portion of a III-V semiconductor first material,on a substrate of a silicon material, comprising: providing a substrateof said silicon material having a surface comprising at least onecatalytic particle on an atomically flat growth region, wherein saidgrowth region is passivated to terminate Si dangling bonds withhydrogen; and epitaxially growing the nanowhisker on said growth regionvia said catalytic particle.
 7. A method according to claim 6, furthercomprising cleaning to remove organic residue from the substratesurface.
 8. A method according to claim 6, further comprising removingoxide formations from the substrate surface.
 9. A method according toclaim 6, wherein said growth region is passivated, at least temporarily,to prevent oxide growth thereon.
 10. A method according to claim 9,wherein the entire substrate surface is passivated.
 11. A methodaccording to claim 6, wherein the catalytic particle is provided and thenanowhisker grown prior to any substantial degradation of thepassivation.
 12. A method according to claim 6, wherein said catalyticparticle is deposited onto the substrate surface by aerosol deposition.13. A method according to claim 12, including annealing the substratesurface at a temperature between about 600 and about 650° C. prior togrowing the nanowhisker.
 14. A method according to claim 6, wherein theIII-V compound is one of GaP, InP, and GaAs.
 15. A method according toclaim 6, including modifying process conditions during growth of saidnanowhisker to provide at least one of a p-n junction and aheterojunction within the nanowhisker.
 16. A method according to claim6, comprising providing a segment of a further material disposed along alength of the nanowhisker, with heterojunctions formed between thesegment adjacent portions of the nanowhisker, so as to provide one of aphotonic device and an electronic device.
 17. A method according toclaim 6, wherein the growth region comprises an entire surface of thesubstrate of the silicon material which is passivated to terminate Sidangling bonds with hydrogen.
 18. A method according to claim 17,wherein the at least one catalytic particle comprise a plurality of goldparticles deposited on the entire surface of the substrate.
 19. A methodfor forming a nanowhisker, having at least a base portion of acrystalline first material, on a substrate of a silicon material,comprising: providing a catalytic particle on a growth region of asubstrate of said silicon material; and epitaxially growing thenanowhisker on said growth region via said catalytic particle, whereinthe substrate is prepared such that the growth region is passivated toterminate Si dangling bonds with hydrogen and which is of a character toenable nucleation conditions that lead to a single nucleus from whichthe nanowhisker nucleates and grows, wherein said first material isselected from a group consisting of II-VI and III-V semiconductorcompounds.
 20. A method according to claim 19, wherein said firstmaterial is a III-V semiconductor compound.
 21. A method according toclaim 19, wherein said at least one catalytic particle is deposited froman aerosol.